Purification of 2,4,5-trichlorophenol

ABSTRACT

2,4,5-Trichlorophenol containing methoxyphenol impurities is purified by reacting the crude material with aqueous chromic acid and separating the purified product.

United States Patent 1 Widiger, Jr.

111 3,760,010 [451 Sept. 18, 1973 PURIFICATION OF 2,4,5-TRlCHLOROPHENOL[75] Inventor: Alexander H. Widiger, Jr., Midland,

Mich.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: July 30, 1970 21 Appl. No.: 59,740

UNITED STATESPATENTS 2,925,361 2/1960 Bollenback 260/623 R 3,499,0453/1970 Cleary 260/623 R Primary Examiner-Bernard l-lelfin AssistantExaminer-W. B. Lone Attorney-Griswold 8L Burdick, Herbert D. Knudsen andC. E. Rehberg [5 7 ABSTRACT 2,4,5-Trichlorophenol containingmethoxyphenol impurities is purified by reacting the crude material withaqueous chromic acid and separating the purified product.

3 Claims, No Drawings PURIFICATION OF 2,4,5-TRICIILOROPIIENOL BACKGROUNDOF THE INVENTION 2,4,5-Trichlorophenol containing various methoxyphenolimpurities is obtained by the caustic hydrolysis of1,2,4,5-tetrachlorobenzene in the presence of methanol. These methoxyimpurities make the product unacceptable for certain high-purity usessuch as the preparation of hexachlorophene. Especially undesirable inthe crude trichlorophenol is the presence of the various isomers ofmonomethoxydichlorophenol. These monomethoxydichlorophenols are believedto form dioxin impurities some of which are ecological and healthhazards.

The separation of methoxydichlorophenol is difficult by conventionalmeans such as distillation and washing. As a result of thesedifficulties, a practical method of separating methoxyphenol impuritiesfrom 2,4,5- trichlorophenol has been sought.

SUMMARY OF THE INVENTION It has now been found according to the presentinvention that methoxyphenols may be readily separated from2,4,5-trichlorophenol by selectively reacting methoxydichlorophenolswith aqueous chromic acid and separating the products from the2,4,5-trichlorophenol by conventional methods, such as washing ordistillation.

The important and novel aspect of the present invention is the selectivereaction of the methoxyphenol impurities in 2,4,5-trichlorophenol byproper adjustment of the amount and concentration of chromic acidemployed, the temperature, time and other reaction conditions. Althoughthe exact nature of the selective reaction of the chromic acid in theinvention is not understood, it has been found that under the properreaction conditions, the amount of undesirable methoxy impurities in thetrichlorophenol can be reduced if not almost completely eliminated. Sucha desirable result and the conditions necessary to obtain this resultare best determined in a specific application by experience. In general,however, as more of the methoxyphenols are present in the reactionproduct, greater amounts and higher concentrations of chromic acid,higher temperatures and longer reaction times are employed to give thedesired reduction of the methoxyphenols in the 2,4,5- trichlorophenol.

The amount of methoxyphenols present in the crude 2,4,5-trichlorophenolmay vary widely depending on the conditions of the caustic hydrolysis ofthe tetrachlorobenzene. The amount of methoxydichlorophenols,

however, is generally less than percent and is usually less than about 5percent. Although the main thrust of the present invention is directedtoward the methoxyphenol impurities, alkoxyphenol impurities obtained bythe caustic hydrolysis of tetrachlorobenzene in other lower alcohols,such as ethanol and propanol, could be reacted in the same manner.

Chromic acid used in the selective reaction of the methoxyphenolspresent in the 2,4,5-trichlorophenol is well known and is generallyprepared in situ by the reaction of an alkali metal dichromate withsulfuric acid. The amount and concentration of the chromic acid may varywidely depending on the concentration and nature of the methoxyphenolimpurities. The concentration of the aqueous chromic acid in thereaction must be adjusted in such manner that at the lower limit,

the acid selectively reacts with at least a major proportion of themethoxyphenol impurities and at the upper limit, the acid will notdeleteriously affect the 2,4,5- trichlorophenol. Although theconcentration of chromic acid within these limits is not critical,aqueous chromic acid solution having a concentration of about 0.5 toabout 10% by weight of chromic acid are preferred.

The temperatures may vary widely during the course of a reaction, withtemperatures above the melting point of the crude trichlorophenol beingof special interest because the reaction is most conveniently conductedin a liquid phase. The temperatures, however, should not exceed about150C. since excessive amounts of by-products are produced by the actionof the chromic acid on the crude trichlorophenol. Preferred aretemperatures of 55 to 100C.

The length of the reaction of the chromic acid with the crudetrichlorophenol may vary widely and is interrelated to the concentrationof the chromic acid and the temperature employed in the reaction. Attemperaturesabove the melting point of trichlorophenol and within thepreferred range of chromic acid concentrations, reaction times of about30 minutes to about 10 hours are generally sufficient to give thedesired reduction in the methoxyphenol impurities.

Although the important and central aspect of the present invention isthe use of chromic acid to remove the methoxydichlorophenols, otheroxidizing agents may be employed in addition to or partial substitutionfor the chromic acid. For example, oxygen or potassium permanganate maybe employed in addition to the chromic acid in the present invention toenhance the methoxyphenol removal or to improve the economics of thereaction. Generally, as more of other oxidizing agents are. employed inthe reaction, less of the chromic acid is required to give resultssimilar to those if no additional oxidizing agent were employed.

SPECIFIC EMBODIMENTS Example 1 In a reactor, 100.0 g. of crude2,4,5-trichlorophenol containing 3.5 percent monomethoxydichlorophenolswas charged and reacted with a solution of 7.5 g. of po tassiumdichromatein 10.0 cc. of concentrated sulfuric acid and 100 cc. ofwater. The temperature of the reactor and contents was maintained at C.for 3 hours.

At the termination'of the reaction, the aqueous phase was separated anddiscarded. The remaining organic phase was washed with 100 cc. of 12percent HCl at 75C., and the aqueous phase was discarded. The wet oilweighed 97 grams and by vapor phase chromatography was analyzed tocontain less than 0.05 percent of monomethoxydichlorophenols. Thus, themethoxydichlorophenols were almost totally eliminated from the crude2,4,5-trich10rophenol.

Example 2 0 1n the same manner as described by Example 1, a reactor wascharged with 100 g. of crude 2,4,5- trichlorophenol containing 3.5percent monomethoxydichlorophenols. ,T his crude 2,4,5-trichlorophenolwas then reacted with 1.0 g. of potassium dichromate in 10 cc. ofconcentrated sulfuric acid and 100 cc. of water at C. During the courseof the 4 hour reaction, a stream of ,oxygen was continuously passedthrough the reaction mixture. The product was recovered by separatingthe aqueous inorganic layer and washing with dilute HCl as described inExample 1. The purified product weighed 104 g. and was analyzed by vaporphase chromatography to contain 2% monomethoxydichlorophenols.

In the same manner as described by the examples above, chromic acid maybe reacted with crude 2,4,5- trichlorophenol containing methoxyphenolsunder different reaction conditions and for different periods of time.For example, the crude 2,4,5-trichlorophenol may be reacted attemperatures of 55 to about 150C. with a suitable amount of about 0.5 toabout percent by weight aqueous chromic acid for times of about ,5 hourto 10 hours or more to substantially reduce or eliminate themethoxyphenols. Also in the same mannet as described by Example 2, otheroxidizing agents, such as potassium permanganate, air and ozone, may beemployed in the reaction to increase the removal of the methoxyphenols.

I claim: 7 I

1. A method of purifying crude 2,4,5-trichlorophenol containing lessthan 10 percent methoxyphenols by weight of the crude2,4,5-trichlorophenol, comprising, reacting the methoxyphenols bycontacting the crude 2,4,5-trichlorophenol with 0.5 to about l0 weightpercent of aqueous chromic acid for about 30 minutes to about 10 hoursat from about 50 to about C. in liquid phase, separating the aqueousphase containing the reacted methoxyphenols and recovering the oil phasecontaining the purified 2,4,5-trichlorophenol.

2. The process of claim 1 wherein the crude 2,4,5- trichlorophenol issimultaneously reacted with a second oxidizing agent.

3. The process of claim 2 wherein the second oxidizing agent is oxygen,air, ozone or potassium permanganate.

2. The process of claim 1 wherein the crude 2,4,5-trichlorophenol issimultaneously reacted with a second oxidizing agent.
 3. The process ofclaim 2 wherein the second oxidizing agent is oxygen, air, ozone orpotassium permanganate.